The present invention generally relates to optical semiconductor devices and more particularly to an integrated optical module that includes a photoreception device and an optical waveguide integrated with each other.
Semiconductor photodetection devices are indispensable in optical information processing systems for use in the field of so-called multimedia where image data and audio data are processed as a part of the information signals processed by the system. In such optical information processing systems, it is essential to achieve efficient optical coupling between an optical waveguide used for transmitting optical signals and a photoreception device used in an optical module for detecting optical signals transmitted through the optical waveguide.
On the other hand, in order that such multimedia is accepted widely by human society, it is necessary to provide the optical processing systems with low cost while achieving efficient optical coupling between the optical waveguide and the corresponding photoreception device.
FIG. 1 shows the construction of a conventional photodetection module proposed previously by the inventor of the present invention.
Referring to FIG. 1, the photodetection module is constructed upon a support substrate 1 that carries thereon wiring patterns 1a and 1b, wherein the wiring patterns 1a and 1b are connected to a semiconductor photoreception device 10 mounted upon the support substrate 1 by way of a flip-chip process.
The photoreception device 10 includes a substrate 2 of n-type InP on which a buffer layer 3 of n-type InP is provided, wherein the buffer layer 3 carries thereon an undoped layer 4 of InGaAs and a layer 5 of n.sup.- -type InP formed on the InGaAs layer 4. Further, p-type diffusion regions 5a and 5b are formed in the foregoing InP layer 5. As a result, it will be noted that pin diodes D1 and D2 are formed in correspondence to the diffusion regions 5a and 5b.
FIG. 2 shows an equivalent circuit diagram of the diodes D1 and D2.
Referring to FIG. 2, it will be noted that the diodes D1 and D2 are connected in series via the n-type InP layer 3 with mutually opposing polarities, wherein the diode D1 forms a drive circuit that drives the diode D2. More specifically, the diode D1 causes a reverse biasing in the diode D2 when the diode D1 itself is forward biased, wherein the diode D2 thus reverse biased in turn causes a conduction in response to an incident optical beam. In other words, the diode D2 acts as a photodiode. It should be noted that the p-type region 5a corresponding to the drive diode D1 has a substantially larger area than the p-type region 5b forming the photodiode D2. Thus, the drive diode D1 can supply a large drive current to the photodiode D2. Associated with such a large area of the p-type region 5a, the drive diode D1 has a large junction capacitance Cp, while the photodiode D2 has a very small junction capacitance associated with the small area of the p-type region 5b. Thus, the photodiode D2 shows a very high response against incident optical beam.
In the photoreception device 10 of FIG. 1, it should be noted that the substrate 2 carries a microlens 2a on the rear side in correspondence to the foregoing photodiode D2, such that the optical beam incident to the rear side of the substrate 2 from an optical fiber 11 is focused upon a part of the InGaAs layer 4 located above the p-type region 5b. Further, the photoreception device 10 includes an insulation film 6 covering the surface of the n-type InP layer 5, wherein the insulation film 6 is formed with contact holes 6a and 6b respectively in correspondence to the diffusion regions 5a and 5b. Further, metal bumps 7a and 7b are formed on the diffusion regions 5a and 5b respectively in correspondence to the contact holes 6a and 6b. Thereby, the photoreception device 10 is mounted upon the support substrate 1 in an inverted or turned-over state by way of a flip-chip process to form the photodetection module, and the metal bumps 7a and 7b are connected electrically as well as mechanically to the foregoing conductor patterns 1a and 1b on the support substrate 1.
In the illustrated construction, it should be noted that another conductor pattern 1c is provided on the rear side of the support substrate 1 in electrical connection with the conductor pattern 1a or 1b by way of a via hole 1d, wherein the conductor pattern 1c is connected to a d.c. current source 12 that supplies a positive voltage. Further, an output terminal and a load resistance R.sub.L are connected to the conductor pattern 1b. As a result, a circuit is formed as indicated in FIG. 2 in which the diode D1 is forward biased and the photodiode D2 is reversely biased. In FIG. 1, it should further be noted that the rear side of the InP substrate 2 carries an anti-reflection film 8.
In the construction of FIG. 1, the photoreception device 10 is mounted upon the support substrate 1 with low cost and with reliability by employing the flip-chip process. Thereby, the fabrication cost of the photodetection module is reduced substantially. Further, by reducing the area of the diffusion region 5b that forms the essential part of the photodiode D2, the response of the photodiode D2 is improved substantially. Further, such a construction of FIG. 1 is advantageous for eliminating mechanical stress from being applied to the active region of the photodiode D2 which is essential for the detection of the incident optical beam. In the structure of FIG. 1, most of the external mechanical stresses applied to the module is absorbed by the diffusion region 5a of the drive diode D1 that has a much larger area than the photodiode D2.
On the other hand, the photodetection module of FIG. 1 has a drawback in that it is necessary to provide a separate holding mechanism for holding the optical fiber 11 in alignment with the microlens 2a on the substrate 2, while such a holding mechanism has to be adjusted for each of the photoreception devices 10 such that an optimum optical coupling is achieved between the core of the optical fiber 11 and the microlens 2a. As the core of an optical fiber has a diameter of about 6 .mu.m at best, such an adjustment of the holding mechanism of the optical fiber 11 takes a substantial time. It should be noted that the adjustment of the fiber holding mechanism has to be made by monitoring the output of the photodiode D2 in search of the optimum position where the output of the photodiode D2 becomes maximum. Thereby, because of the long time needed for the adjustment, the fabrication cost of device increases inevitably in the optical module of FIG. 1.
On the other hand, there are proposals for optical modules that does not require such a holding mechanism of optical fiber as indicated in FIG. 3, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 3, there is provided an optical waveguide 13 on the upper major surface of the support substrate 1, wherein the optical waveguide 13 is formed monolithically upon the substrate 1 and includes a waveguide layer 13c sandwiched vertically by a pair of cladding layers 13a and 13b. Further, a mirror element 14 having a mirror surface 14a is provided in the path of the optical beam emitted from an edge surface 13A of the foregoing waveguide layer 13c. It should be noted that the mirror element 14 has a lower major surface contacting to the upper major surface of the support substrate 1 and an upper major surface parallel to the foregoing lower major surface, and the photoreception device 10 is carried upon the upper major surface of the mirror element 14.
In such a construction, the optical beam guided through the optical waveguide 13 and emitted from the edge surface 13A, is reflected by the mirror surface 14a and impinges upon the rear surface of the substrate 2 of the photoreception device 10, wherein the optical beam thus entered into the substrate 2 reaches the active region of the photodiode D2 that corresponds to the diffusion region 5b.
Thus, by providing the mirror element 14 at a predetermined position of the support substrate 1 and by providing the photoreception device 10 at a predetermined position of the upper major surface of the substrate 1, it is possible to achieve an optical coupling between the optical waveguide 13 and the photodiode D2 of the photoreception device 10 easily, by merely aligning the photoreception device 10, the mirror element 12 and the optical waveguide 12 with each other on the substrate 1. It should be noted that such a positional alignment may be achieved easily by forming alignment marks on the substrate 1 or on the mirror element 14.
The construction of FIG. 3, however, has a drawback in that it requires the mirror element 14 as an additional component. Associated with this, the construction of FIG. 3 requires additional fabrication steps. Further, use of such a mirror element 14 tends to cause modification of the optical beam path in the optical integrated circuit. In order to achieve an optimum optical coupling between the photodiode D2 and the optical waveguide 13, it is necessary to adjust both the mirror element 14 and the photodetection device 10 with respect to the optical waveguide 13, while such an adjustment is extremely difficult particularly when the precision of the mirror element 14 is insufficient.
Further, use of such a mirror element 14 in the optical path of the optical beam tends to increase the optical path length of the optical beam emitted from the edge surface 13A of the optical waveguide 13. When the optical path length of the optical beam is increased, the optical beam experiences a substantial beam divergence when it reaches the active region of the photodiode D2. In such a case, it is necessary to increase the size of the active region of the photodiode D2 in correspondence to the increased beam diameter of the optical beam, while such an increase in the size of the active region of the photodiode D2 invites an increase in the junction capacitance of the diffusion region 5b that forms the active region of the photodiode D2. Thereby, the response of the photodiode D2 is inevitably deteriorated. When the size of the active region 5b of the photodiode D2 is to be set small for maintaining high response speed, on the other hand, the efficiency of optical coupling of the photodiode D2 is degraded substantially due to the fact that most part of the optical beam from the optical waveguide 13 misses the active region 5b of the photodiode D2.
FIG. 4 shows the construction of another conventional photodetection device, wherein those parts described previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 4, it will be noted that the photoreception device 10 is now flip-chip mounted directly upon the upper major surface of the support substrate 1 that carries also the optical waveguide 13 thereon as an integral, monolithic body. In such a construction, the edge surface of the active layer 4 formed on the InP substrate 2 faces the exposed edge surface 13A of the waveguide layer 13c, and the optical beam emitted from the edge surface 13A directly enters into the active layer 4. Thereby, the problem of increased optical path distance of the optical beam is successfully eliminated.
On the other hand, such a construction also has a drawback in that most part of the optical beam emitted from the optical waveguide layer 13c, which typically has a thickness of about 6 .mu.m, misses the active layer 4 that has a thickness of only 2-3 .mu.m. It should be noted that the optical beam emitted from the optical waveguide layer 13c has a size corresponding to the thickness of the layer 13c in the vertical direction. In other words, the construction of FIG. 4 inherently provides a large optical loss and cannot provide a satisfactory optical coupling.
In addition, the construction of FIG. 4 requires adjustment process between the photodetection device and the external optical waveguide layer 13 on the substrate 1, for maximizing the optical coupling between the active layer of the photoreception device 10 and the optical waveguide 13, while such an adjustment is complex and increases the cost of the optical module.